Adjustable resistor



Filed Sept. 28, 1962 N- y I V Y- f fi; INVENTOR. Joseph Bebel United States Patent O 3,233,200 ADJUSTABLE RESISTOR Joseph Bebel, Lakewood, NJ., assignor to Weston Instruments, Inc., a corporation of Texas Filed Sept. 28, 1962, Ser. No. 226,875 16 Claims. (Cl. 338-180) This invention relates to an adjustable electrical resistor wherein the resistance elements in the for-m of an electrically conductive film and, more particularly, to potentiometers using electrically conductive films of relatively low resistance values.

It is known to form adjustable electrical resistors in which the electrically resistive element consists of a carbon or metal film deposited by sublimation, evaporation, or other known techniques upon the inside of a tubular substrate or base member, In such resistors the inside of the tubular substrate usually is glazed prior to metal deposition. Terminations `of an electrically conductive material are provided on the tube ends and a portion of the outside of the tube at each end. These terminations provide the necessary electrical path from the resistive element to the end caps which are force fitted on the outer tube diameter.

The resistive element is adjusted to a desired resistance value by a method known as spiralling Spiralling is accomplished through the use of an electric pr-obe or an abrasive wheel to form a helical cut in the resistive element -on the inside of the tubular substrate such that the resistive path is in the form of a helix or spiral. The resistance between the ends of the tubular substrate may be varied by changing the pitch of the spiral.

In units of relatively low resistance, however, unspiralled resistance elements often are used. When such low resistance elements are used, the typical point contact wiper causes a non-linear and unstable wiper displacement vs. resistance characteristic because of field effects. Field effects is the term used to describe that phenomenon wherein the electrical current -flows from the contractor or wiper radially into the resistive lm in all directions before traversing the film and converging upon the other electrical termination. This radial distribution of the several current paths tends to increase the resistance offered 4to current flow. The problem is so severe that in a typical resistance element, the field effects can produce a much higher contractor-to-termination resistance than that existing between the electrical terminations on the tube ends. As such, the unit is non-linear and virtuallyy useless as a variable resistor. In spiralled units the current path generally is relatively narrow, i.e., in the order of .03 inch wide. With such narrow current paths, there is little opportunity for radial current flow. Hence the problem of field effects generally is not encountered in spiralled units.

The problem of field effects can be overcome by the use of a multiple contactor instead of a single contactor. Various types of multiple contactors have been employed in the prior art. `One such type involved the use of a circular piece of wire intended to contact virtually all points of the inner circumference of a film coated tubular base member. This type contactor has not been entirely satisfactory. Due to the normal surface irregularities of ice sensitivity is due to the pressure differentials arising as the multiple contactor is either pushed or pulled along the film. Pulling the leaf spring contactor along the film lightens its pressure against the film and greatly increases noise or contact resistance. If the contactor is pushed, the converse is true, i.e., contact pressure increases which greatly decreases contact resistance. To overcome this directional sensitivity, the contactor length must be greatly reduced to produce translational forces that are tangential (not normal) to the film surface. Unfortunately such shortening greatly increases contact pressure and hence film wear.

Also some difficulty has been encountered in designing a contactor that is capable of maintaining constant film contact pressure with variations in the smoothness of the surface on which the film is deposited.

It is, therefore, an object of this invention to obviate many of the disadvantages of the prior art adjustable resistors.

Another object of this invention is to facilitate the positioning of an electrical contactor on an electrically conductive film.

Still another lobject of this invention is to provide an improved contactor for film type adjustable resistors.

In a preferred embodiment of this invention the adjustable resistor consists of an electrically non-conductive tubular base member which has a thin film of an electrically conductive material deposited on its inside wall. Terminals are placed at either end of the tubular base member so as to contact at least a portion of the thin film. End caps having openings are placed over either end of the tubular base member. The openings yin the end caps are such as to coaxially position within the base member a contactor support or slider threaded on a lead screw.

In accordance with this invention a contactor formed of a helical wire toroid is mounted in a groove in the contactor support. The c ontactor support is spool-like with the contactor groove inclined with respect toits axis. The depth of the groove and the diameter of the wire helix forming the toroid are selected such that the toroid outer diameter is slightly greater `than the maximum inner diameter of the tubular basemember. Contact pressure between the turns lof the wire forming the toroid and the film resistance element is obtained by the fall-over of the coil' turns of the toroid lwhen inserted within the tubular base member. A

The frictional force of this pressure overcomes the Arotational force of the lead screw. Due to this frictional force, rotation of the lead screw advances the contactor longitudinally along the resistance element. This arrangement has many advantages only one of which is that it provides a multipoint contact which greatly reduces the ..field effects.

Further advantages and features of this invention will become apparent upon consideration of the following description read .in conjunction with the drawing wherein:

FIGURE l is an isometric view of an adjustable resistor .formed in accordance Iwith this invention;

FIGURE 2 is an axial elevational cross-sectional view of the adjustable resistor illustrated in FIG. 1;

FIGURE. 3 is an end partial-section view of the adjust-able resistor illustrated in FIG. 1 taken along the partial-section line 3-3 of FIG. 2; and

FIGUR-E 4 is a perspective view of the slider of this invention.

The adjustable resistor or potentiometer shown in FIGS. 1 through 4 comprises a tubular ceramic base member 10 which may be formed of an electrically nonconductive material such as a ceramic. The adjustable resistor also includes a lead screw 12 on which is threaded a follower 14. The follower 14 and lead screw 12 comprise the adjusting mechanism of the resistor. The adjusting mechanism is mounted by suitable end and header assemblies 16 and 18. vThis entire unit, including the base member 10, the adjusting mechanism and the end and-header assemblies 16 and 18, respectively, is encapsulated and sealed by an epoxy resin. A terminal board 22 is placed adjacent the outside of the base member 10 prior to encapsulation to provide for an external electrical connection .to both ends and to the follower of the adjustable resistor through pins 30, 31 and 32 in the terminal board 10.

Having thus described the overall configuration of the adjustable resistor, the specific details 4of its construction in accordance with this invention will now 'be described with particular refe-rence to FIGS. 2, 3 and 4.

The tubular base member 1t) consists of an electrically non-conductive tube on the inside of which is deposited `a thin film 26 ofA an electrically conductive material, i.e., a resistance lm. The resistance film 26 may be .any of the 'known lms such as a carbon film or a nickel composition deposited by sublimation, evaporation, or other known methods upon the ends and inner diameter of the tubular base member 10. Typically the inner diameter of the tubular base member is glazed prior to metal deposition. Terminations 24 of an electrically conductive material such as a metallic frit or a gold plating are provided on the ends over the film 26 and on a portion of the outer diameter of each end of the tubular base member 10. These terminations 24 provide the necessary electrical path from the ends of film 26 to end caps 23 and 29. The end caps are metallic and are force fitted over the ends of the base member 1t) to provide such electricalv path. An external electrical connection to the end caps 28 and 29 is made through the respective pins 30 and 32 of the terminal board 22 to terminals 34 which are soldered to the respective end caps 2S and 29.

The end caps 2-8 and 29 are generally in the shape of cups having central openings therein to receive and mount the adjusting mechanism of the adjustable resistor. The adjusting mechanism includes the header assembly 18, and the end assembly 16 which mount and centrally locate the lead screw 12 coaxially within the base member 10. The header assembly 18 is made up of a slotted adjustment he-ad 36 .and a drive bushing 3S which are assembled into an integral unit by the use .of a disc-like molding 40. The adjustment head 316 and drive bushing 38 are 'of a suitable metallic material whereas the molding 40 may be of .a suitable pl'asticsuch as Teon. The molding 40 aids in sealing the interior of the resistor. fPreferably it has a low c-oefcient of lfriction to permit its rotation within the end cap 29. Also it desirably should .be some- 'what exible and non-conductive to provide electrical insulation between the adjustment head 36, and lthe'drive bushing 3S and the end cap 29.

The header end of the resistor is sealed by the use of elastic washers 56 .placed on either side of the molding 40 and an additional bushing 52 which abuts the end of the resistor. This additional bushing 52 reduces wear on the Amolding 4t) and aids in centrally locating the 'bushing 38.

The lead screw 112, whose lcenter portion is threaded, may be gold plated for improved conductivity and corrosion resistance, and is fixedly coupled at one end to the ldrive bushing 38. Rotation of the slotted adjustment head 36 thus causes rotation of the lead screw 12. The other end of the lead screw 1-2 is rotatably supported by Vthe end assembly 16 which includes the drive bushing 42 coaxially and fixedly supported within an outer bushing 54. The outer bushing 54 is lof an electrically non-conductive material. A terminal is soldered to the xed bushing 42. The fixed bushing 42 is of a suitable metallic material and may have its bearing surface gold plated for improved electrical conductivity to the lead screw 12 so as to con-duct current to the terminal 35. The terminal 35 is electrically connected (by means not shown such as a wire) to the center pin terminal 34a and hence to the 4center pin 31 on the terminal board 22.

The follower 14 is normally made of any suitable material such as brass and preferably is rhodium plated for improved electrical conductivity, wear resistance, and corrosion resistance. It is threaded to allow mating with the thread 'of the lead screw 12. The outer bushing 54 a-cts as a seal and also an electrical insulator between the fixed bushing 42 and the left hand end cap 2S. This end cap is force fitted over the left end (in the drawing) of the -base member 10 to provide the electrical connection to the pin 3? from the left end of the film 26. j

In accordance with the invention, the follower 14 is spool-like with a contactor groove 46 about its circumference. The groove 46 lies in a plane that is inclined with respect to the axis of the spool-like or cylindrical shaped follower 14. The contactor comprises a coil having a plurality of turns of a fine wire formed into a toroid and placed within the circular groove 46 of the follower 14. The toroidal contactor 44 is formed with its outer diameter substantially equal to or -slightly greater than the maximum inner diameter of the base member 1t). In this manner the turns of the coil forming the toroidal contactor '44 are forced over slightly, which may be termed fallover, when they are inserted into the base member 1t). The resulting pressure on the lm caused by the fiexure or fall-over of these turns of the toroidal contactor 44 overcomes the rotational force of the lead screw adjustment. Hence, rotation of the lead screw 12, by rotation of the slotted adjustment head 36, advanced the toroidal contactor 44 and follower 14 laterally along the length or axis of the resistance element 26.

Aspring 70 and a pair of washers 72 placed on either side of the spring 7d are placed on both ends of Athe lead screw 12 to provide a clutch action. The washers may be gold-plated brass and the spring beryllium copper. The spring is placed into compression by the inner washers 72 which are located upon shoulders 74 formed at either end of the lead screw 12. These spring and washer combinations act as free running clutches for the follower 14. The lead screw 12 is not threaded at either fend to permit the clutch action and the ends (beyond the shoulders 74) are of reduced diameter so that the follower 14 may disengage from the threads as it reaches either end of the film 26. As the follower 14 disengages, it compresses either of the springs 70 such that upon reversal of rotation of the adjustment head 36, the compressed spring 70 forces reengagement of the follower 14 with the threads of the lead screw 12. The clutch action prevents damage to the unit through excessive rotation of the adjustment head 36.

This particular configuration has many advantages. First of all, it provides a multi-point contactor in which a separate contact exists at each turn of the toroidal contactor 44. These several points of contact with the film 26 substantially reduces lfield effects. The multiplicity of contactors improves reliability in that if one or more turns of the toroid 44 become coated or otherwise develop a higher resistance their effect is small. Additionally, the multiplicity of contactors reduces noise and contact resistance.

The voffset positioning of the groove '46 and hence the toroidal contactor 44 in the follower 14 aids in maintaining a positive contact between the film and the toroid contact and in maintaining a positive contact between the follower 14 and the lead screw 12. The fall-over of the toroidal turns provides a light but flexible contact with the film that conforms to any unevenness of the film surface or diameter of the base member 1t). The need for additional guides to prevent rotation of the contactor 44 is eliminated by the pressure contact provided by the toroid. The pressure of the toroidal contactor 44 on the film 26 is normal to the film surface and hence the contactor is not directionally sensitive. By the continuous movement -of the toroidal contactor over the film, a very high degree of resolution is obtained.

Having now described my invention in detail, in accordance with. the patent statutes, various changes and modifications willsuggest themselves to those skilled in this art, and it is intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the ltubular base member;

a slider adapted to move through the interior of the tubular base member in a direction substantially parallel to the axis thereof;

and an electrically conductive toroidal coil mounted on the slider for providing a multi-point electrical contact between the slider and the resistive element, the coil being mounted so that the plane of the toroid define-d by such coil is at Van angle effectively different from 90 with respect to the axis of the tubular base member.

2. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the tubular base member;

a slider adapted to move through the interior of the tubular base member in a direction substantially parallel to the axis thereof;

and an electrically conductive toroidal coil mounted on the slider for providing a multi-point electrical contact between the slider and the resistive element, the coil being mounted so that the plane of the toroid defined by such coil is at an angle at least 1 different from 90 with respect to the axis of the tubular base member.

3. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the tubular base member;

a slider adapted to move through the interior of the tubular base member in a direction substantially parallel to the axis thereof;

and an electrically conductive toroidal coil mounted on the slider for providing a multi-point electrical contact between the slider and the resistive element, the coil being mounted so that the plane of the toroid defined by such coil forms an acute angle 'with the axis of the tubular base member.

4. An adjustable resistor comprising:

an electrically.non-conductive tubular base member;

a thin electrically resistive film on at least a portion of the inside wall of the tubular base member;

a slider adapted to move through the interior of the tubular base member in a direction substantially parallel to the axis thereof;

and van electrically conductive toroidal coil mounted on the slider for providing a multi-point electrical contact between the slider and the resistive film, the coil being mounted so that the plane of the toroid defined by such coil is at an angle effectively different from 90 with respect to the axis of the tubular base member.

5. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the tubular base member;

a slider adapted to move through the interior of the tubular base member in a direction substantially parallel to the axis thereof;

and an electrically conductive toroidal coil mounted on the slider for providing a multi-point electrical contact between the slider and the resistive element, the outside diameter of the toroid defined by such coil being greater than the inner diameter of the tubular base member and the coil being mounted so that the plane of the toroid is at an angle effectively different from with respect to the axis of the tubular base member.

6. An adjustable resistor comprising:

`an electrically non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the tubular base member;

a cylindrical slider adapted to move through the interior of the tubular base member in a direction substantially parallel to the `axis thereof, the periphery of the slider having a groove therein which encircles the axis of the slider, this groove being in a plane which is at an angle effectively different from 90 with respect to the axis of the slider;

and an electrically conductive toroidal coil mounted in the groove on the slider for providing a multi-point electrical contact between the slider and the resistive element.

7. An adjustable resistor comprising:

an electricallyv non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the tubular base member;

a threaded lead screw rotatably mounted coaxially within the tubular base member;

a cylindrical slider threaded on the lead screw;

and an electrically conductive toroidal coil mounted on the periphery of the slider for providing Aa multi-point electrical contact between the slider and the resistive element, the coil being mounted so that the plane of the toroid defined by such coil is at an angle effectively difierent from 90 with respect to the axis of the tubular base member.

8. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on Iat least a `portion of the inside wall of the tubular base member;

a threaded lead screw rotatably mounted coaxially within that tubular base member;

a cylindrical slider threaded on the lead screw;

Iand an electrically conductive toroidal coil mounted on the periphery of the slider for providing a multipoint electrical contact between the slider and the resistive element, the coil being mounted so that the plane of the toroid defined by such coil is at an angle at least 1 different from 90 with respect to the axis of the tubular base member.

9. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the tubular base member;

a threaded lead screw rotatably mounted coaxially within the tubular base member;

a cylindrical slider threaded on thelead screw;

and an electrically conductive toroidal coil mounted on the periphery of the slider for providing a multipoint electrical Contact between the slider and the resistive element, the coil being mounted so that the plane of the toroid defined by such coil forms an acute angle with the axis of the tubular base member.

10. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

a thin electrically resistive film on at least a portion of the inside Wall of the tubular base member;

a threaded lead screw rotatably mounted coaxially within the tubular base member;

a cylindrical slider threaded on the lead screw;

and an electrically conductive toroidal coil mounted on the periphery of the slider for providing a multipoint electrical contact between the slider and the resistive film, the coil being mounted so that the plane of the toroid defined by such coil is at an angle effectively different from 90 with respect to the axis of the tubular base member.

11. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the tubular base member;

a threaded lead screw rotatably mounted coaxially within the tubular base member;

a cylindrical slider threaded on the lead screws;

and an electrically conductive toroidal coil mounted on the periphery of the slider for providing a multipoint electrical contact between the slider and the resistive element, the coil being mounted so that the plane of the toroid deined by such coil is at an angie eiectively different from 90 with respect to the axis of the tubular base member, the outside diameter of the toroid being greater than the inside diameter of the tubular base member and such that the turns of the coil are caused to ex and exert a pressure on the resistive element whereby the frictional force of the coil prevents rotation of the slider during rotation of the lead screw.

12. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on at least :a portion of the inside wall of the tubular base member;

a threaded lead screw rotatably mounted coaxially within the tubular base member;

a cylindrical slider threaded on the lead screw;

an electrically conductive toroidal coil mounted on the periphery of the slider for providing a multipoint electrical contact between the slider and the resistive element, the coil being mounted so that the plane of the toroid defined by such coil is at an angle effectively dierent from 90 with respect to the axis of the tubular base member;

and means for connecting at least one end of the resistive element and the slider into an electrical circuit.

13. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the tubular base member;

a threaded lead screw rotatably mounted coaxially within the tubular base member;

a cylindrical slider threaded on the lead screw, the periphery of the slider having a groove therein which encircles the axis of the slider, this groove being in a plane which is at `an angle efrectively different from 90 with respect to the taxis of the slider;

and an electrically conductive toroidal coil mounted in the groove yon the periphery of the slider for providing a multi-point electrical contact between the slider and the resistive element.

14. An adjustable resistor comprising:

an electrically non-conductive tubular base member;

an electrically resistive element on at least a portion of the inside wall of the tubular base member;

a threaded lead screw rotatably mounted coaxially within the tubular base member;

a cylindrical slider threaded on the lead screw;

an electrically conductive toroidal coil mounted on the periphery of the slider for providing a multipoint electrical contact between the slider and the resistive element, the coil being mounted so that the plane of the toroid defined by such coil is at 'an angle effectively different from with respect to the axis of the tubular base member;

at least one end of the lead screw being unthreaded for enabling disengagement of the slider from the threads of the lead screw adjacent a corresponding end of the resistive element;

:and spring means including a spring member coaxially positioned over the Unthreaded end of the lead screw so that the slider compresses the spring member as it leaves the threaded portion of the lead screw, such compression enabling reengagement of the slider on the threads of the lead screw upon a rever'- sal of its rotational direction.

15. An adjustable resistor comprising:

a tubular resistive element;

-an electrically conductive toroidal coil adapted to contact the surface of the resistive element;

and means for moving the toroidal coil axially along the resistive element while maintaining the plane of the toroid defined by such coil at an acute angle with respect to the axis of the resistive element.

16. An adjustable resistor comprising: v

a tubular resistive element;

an electrically conductive toroidal coil adapted to contact the interior surface of the resistive element;

and means for moving the toroidal coil axially through the tubular resistive element while maintaining the plane of the toroid defined by suchcoil at an acute angle with respect to the axis of the resistive element;

the outside diameter of the toroid defined by such coil being greater than the inside diameter of the tubular resistive element and such that the turns of the coil are caused lto ex and exert a pressure on the resistive element.

References Cited by the Examiner UNITED STATES PATENTS 2,178,241 lO/l939 Rubenstein 338--- 2,870,304 1/1959 OBrian v3238-180 X 2,895,116 7/1959 Morrison M 338-180 2,898,569 8/1959 Royce 338-l80 2,954,539 9/1960 Blanco 338-180 X RICHARD M. WOOD, Primary Examiner.

dit, 

15. AN ADJUSTABLE RESISTOR COMPRISING: A TUBULAR RESISTIVE ELEMENT; AN ELECTRICALLY CONDUCTIVE TOROIDAL COIL ADAPTED TO CONTACT THE SURFACE OF THE RESISTIVE ELEMENT; AND MEANS FOR MOVING THE TOROIDAL COIL AXIALLY ALONG THE RESISTIVE ELEMENT WHILE MAINTAINING THE PLANE OF THE TOROID DEFINED BY SUCH COIL AT AN ACUTE ANGLE WITH RESPECT TO THE AXIS OF THE RESISTIVE ELEMENT. 